Wednesday, 28 March 2012

Crankcase Breather

Ok so I have a heap of oil all over the back rim which is coming off the chain.  I also have some coming out of the left side and building up on the fin around the spark plug.  It seems all this can be linked back to the breather box.  There were at least three variations to the breather box and yamaha never seemed to get this right from what I have been reading. Originally it had the larger box with 2 outlets, later the sealed one of those outlets and just ran one hose. Later on they went to the smaller box with a single narrow outlet which I hear was too restrictive.

I have excessive oil which could mean it is overfilled (see note on wrong capacities lower down) but I doubt this as the bike hasn't had an oil change since I got it. It is on the to do list.  So there is a possibility that the baffles/fins are gummed up.  Also worth noting that mine is a 74 with the larger box and one hose running down to oil the chain. It is just getting far more oil than it should. The second un used nipple is filled with what looks like silicon. If it is and the PO was a bit over zealous with the silicon there could be the source of a blockage.

My intention is to machine up a new single nipple and swap it for the twin stock nipple. When removed    inspect the baffles.

Restrictive (short) late style breather (left) and the low restriction early model long dual outlet breather (right), it has two 3/8 inch outlet holes and a double the stock size oil separator metal baffle hole inside. This big assembly causes carb mounting interference problems and Yamaha discontinued it, but the smaller replacement design it not big enough for a stock motor, much less a bored, performance tuned or stroked motor.
The small holes in the stock breather housing and stock baffle plate shown above can cause the cam chain tensioner rectangular cover gasket, the cylinder base gasket, the clutch push rod seal, and the countershaft output shaft seal to blow and or seep or leak oil due to excessive crankcase pressure.

On the early models the breather vent emptied out over the chain, helped keep it oiled.  Obviously the will be some oil/mist leakage on an engine with excessive wear/blowby. As the years went by they changed the breather system. The epa didn't like oil blowed all over. They ended up with the breather vented into the airboxes and sucked into the engine to be burnt. 

Some guys are putting filters, oil tins, etc to catch the misted oil. Others are putting in a break valve. You can't use a PCV valve because they have a ball bearing in them and it won't move fast enough at higher RPMs. Basically, the thing won't function. Needs to be the reed or flap type.

 on my 74 I ran long tubes down the back frame tube to dump under the engine with the check valves tucked between the starter and swingarm area and yes the 74's say 2500cc but are happier with 2300cc they were around the time of the case capacity changeover and Yamaha dealers were remarking the oil dipsticks per a service bulletin 

 First, I'm very careful to run the proper amount of oil. I set the level about half way between the low and high mark just as Yamaha recommends. Second, and I feel this is the most important thing, I have the factory rubber restrictor plugs in place in the ends of the breather nipples. Run without those and you will mist out more oil, regardless of the level.

 old motors were 2000cc late motor 2300cc you have 1 early dipstick yam dealers remarked a lot of them as they changed the case capacity before they changed the dipstick  

 Yamaha got complaints about excess oil coming out the breather and revised the oil level in '75. I don't think the cases ever changed, just the amount of oil you put in them (less on later models), hence the dip stick marking change. That amount stamped on the case is for a new totally empty motor or for one after a complete teardown. You won't get that much oil in there without overfilling it on a routine oil change. And if you overfill these motors, it WILL blow out the breather (DAHIK). Here's a service bulletin …..

 Incidentally... how many of you lot actually run the little cork dipstick cover gasket? I don't... never knew there was one until recently... but I wonder if that might make it even easier for the breather valves to make some vacuum. I'd think that every little bit helps. Mike's has them, but that's a gasket that's easily made by oneself. I'll have to get/make one. 

 Positive crankcase pressure can be caused by blow by piston rings bore clearence etc...thats what causes it when there is something wrong... WILL get positive pressure however slight in a well sorted motor if you remove your original airbox and fit pod filters. 
After weeks of research and exchanging emails with a guy in Australia who is an expert on crankcase breathing in many different types of engine I learnt the issue...
We run a motor that has two pistons that arrive at the top both at once, causing no natural equalization of pressure. They are two quite large pots in a small space...taking off the induction breather system means that the original breather system now becomes a slight just fitting a straight through breather system becomes say 50% effective in comparison to the inducted breather...
Natural blow by (which is unavoidable even in the most well sorted new engine) means that there is always more energy causing positive pressure than there is negative. Therefore with a straight through breather that has the same resistance to output as input you get positive pressure whether you like it or not. 
The ONLY way round this is to fit the airbox fit very very freeflowing one way valves..
The airbox is smell free because its a sealed circuit...and lets say this is 100% effective as mostly it is...if thats try a well sorted one way valve setup is say 80% effective ...but it still smells. 
I had a leak from my cam caps..(what I call them) and fitting the one way valve from Mikes which is easily opened...fixed it ..straight away..
Before that I had break servo valves that allowed too much positive pressure build before they opened..
Mikes valve is a fast moving light flap of material .. 

 I have found that if you just put one in the end of the a long hose and let it set behind the engine it can get dirty. I just check it and spray it out with a blast of carb cleaner. 
If you use a short hose to hokk the check valve close to the head and run a long hose down from there the valve stays clean.

Interesting side note on the oil levels the book says to check oil warm, on the center stand, let set a few minutes, remove dipstick wipe clean, set back in hole with dipstick just setting on threads, not screwed in. This is a problem for me as I don't have a centre stand!!!!

Never heard anyone mention steel wool before, is it correct from factory?? 
The stainless steel wool in there is acts as an oil seperator. Oil vapour passing through it will condense on the wool as it goes past and drain back to the sump. It it required and what grade wool?

I have an old double spigot breather from an older engine here...took it apart...there is a plate that covers the vent holes inside and under that the factory stuffed steel wool in it....the wool had hardended goo down deep and I think it was plugging it up.........sure would have positive pressure if that was the case and would could make seals leak....the metal cover inside is hard to get out though...….

 The valve I made yesterday was put in this morning......noticable better running....hard to describe but the engine feels less encumbered as its tooling along......sounds like a couple few more MPG and it pulls off noticably better with less throttle.....and pulls my high 5th better too....not the first valve I've made but the first for this bike…...


Ok since I'm wasn't sure what I would be dealing with and can't afford to have my only source of freedom aka transport off the road. I decided to pull the box off the 76 motor which is out of the frame. Easy access!

The nipple 'Y' bend is a press fit so was relatively easily knocked out, after removing the baffles. After measuring it up I found I didn't have any aluminium stock the correct size to machine the new single.  I will make up at least 5 of them so I can do this for all the bikes and have a spare. 
In the mean time I decided to clean the box up a bit with some wet and dry.

Not 100% sure if this baffle should have the hole to the top or bottom. I think it is the bottom or the 90Deg. piece on the back ends up in the middle of the nipple on re assembly.  Clarification on this would be appreciated.

The breather box that attaches to the cylinder head has some strange baffle system to separate oil from the "breath" and return it to the engine.

It does never work perfectly. The oil content of the "breath" is still quite high - you'll notice it by any longer ride ;-) 

So for me I will get some stock to machine a single nipple, get the baffles in, without steel wool, and swap this on to the bike and the redo the take off for the 76. I will try and check the oil level to mid point on the dipstick. Then reassess as to if I want to add the valve in.

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